1. Field of Invention
The present invention relates generally to a Hybrid Electric Vehicle (HEV), and specifically to an HEV system controller that determines xe2x80x9cengine onxe2x80x9d by measuring variations in crankshaft speed.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles powered by an Internal Combustion Engine (ICE) is well known. Vehicles powered by electric motors attempt to address these needs. However, electric vehicles have limited range and limited power capabilities and need substantial time to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called Hybrid Electric Vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 (Severinsky).
The HEV is described in a variety of configurations. Many HEV patents disclose systems in which an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a Series Hybrid Electric Vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A Parallel Hybrid Electrical Vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that together provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A Parallel/Series Hybrid Electric Vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is typically known as a xe2x80x9cpowersplitxe2x80x9d configuration. In the PSHEV, the ICE is mechanically coupled to two electric motors in a planetary gearset transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque powers the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery if a regenerative braking system is used.
The desirability of combining an ICE with an electric motor is clear. The ICE""s fuel consumption and emissions are reduced with no appreciable loss of vehicle performance or range. Nevertheless, there remains a substantial opportunity to develop ways to optimize HEV operation.
One such area of development is determining if the engine is on. In a conventional vehicle, xe2x80x9cengine onxe2x80x9d status can be easily determined after xe2x80x9ckey onxe2x80x9d by comparing the actual engine speed to a threshold value that indicates the engine is producing torque and combustion. It can also be determined by simply listening for engine noise or feeling engine vibration. However, in an HEV the engine may not be running after xe2x80x9ckey onxe2x80x9d and sometimes not even when the vehicle is in motion. Therefore, it becomes necessary for the Vehicle System Controller (VSC) to identify xe2x80x9cengine onxe2x80x9d status before making powertrain torque determinations.
The prior art has disclosed methods to determine the xe2x80x9cengine onxe2x80x9d status. Unfortunately, these methods often relate specifically to conventional ICE vehicles. For example, in U.S. Pat. No. 5,372,101 to Hoshiba, et al., engine speed is measured to determine if the engine is starting or running. This method does not work with an HEV because the HEV""s generator can spin the engine, thus producing engine speed without combustion occurring. Therefore, engine speed in this situation is not a reliable measurement of xe2x80x9cengine onxe2x80x9d status in a HEV.
In U.S. Pat. No. 5,601,058 to Dyches, et al., a method of measuring starter motor current is disclosed and in U.S. Pat. No. 6,009,369 to Boisurart, et al., a method of measuring alternator voltage is disclosed to determine if the engine is running. These two methods are also inapplicable to the HEV because the HEV does not use a conventional starter motor or alternator.
Therefore, it is necessary to develop a way for the VSC to determine xe2x80x9cengine onxe2x80x9d status in HEVs before allowing engine torque request.
Accordingly, the present invention provides a method and system for determining the xe2x80x9cengine onxe2x80x9d status in an HEV.
The HEV relies upon the generator motor to spin up or xe2x80x9cmotorxe2x80x9d the engine. Therefore, it is not possible to measure engine speed to determine whether the engine is running. It is an object of the present invention to provide a reliable method to determine xe2x80x9cengine onxe2x80x9d status in an HEV by measuring variations in crankshaft speed. The Vehicle System Controller (VSC) monitors engine speed in a conventional manner known in the prior art (e.g., Hall Effect sensor), but rather than using absolute engine speed to determine engine status, the VSC looks for engine speed variations caused by the periodic nature of the combustion process in an IC engine. D. Taraza, et al, in Determination of the Gas-Pressure Torque of a Multicylinder Engine from Measurements of the Crankshaft""s Speed Variation, SAE 980164 (1998), performs a discrete Fourier transform on the crank speed signal, then uses the amplitude of the 3rd harmonic to determine engine torque. This invention does not need to go as far as predicting exact torque, only the difference between a motoring engine and a xe2x80x9crunningxe2x80x9d engine. A motoring engine will have very little speed variation because electric motors have a very smooth torque output. Once the VSC determines the engine is running, it can allow engine torque requests.
A system to perform the above method comprises a controller, an engine, a generator, and measuring device to determine crankshaft speed. The controller determines the need for the engine to be on, starts the engine, and then determines xe2x80x9cengine onxe2x80x9d status by comparing crankshaft speed variation to a calibratable threshold.